Blind Transport Format Detection Depending on the Conditions of Reception of the Signal

ABSTRACT

A method for determining a length of a message block of k bits from a length candidate set, comprising steps of: —Selecting ( 401 ) a length candidate (N) among the set; —Decoding ( 402 ) a received frame to form a decoded sequence that includes a message of a length equal to the length candidate (N), by a Viterbi decoder; —Calculating ( 402 ) a Viterbi variable (S(N)) for this candidate (N), —Comparing ( 403 ) the Viterbi variable with a threshold (Δ), —Repeating the selecting ( 401 ), decoding ( 402 ) and calculating ( 402 ) steps if said Viterbi variable (S(N)) is greater than the threshold (Δ) and if there exists an unselected length candidate among the set; —If the Viterbi variable is greater than a best value (S best ), updating this best value to the Viterbi variable and updating a best length (N end ) to this candidate; —Wherein the threshold (Δ) is initially set to a value determined according to the conditions of reception of the receiver.

FIELD OF THE INVENTION

The present invention generally relates to the field of datatransmission technology for communication systems. More particularly, itrelates to a system and a method for blind transport format detectionwith cyclic redundancy check.

BACKGROUND OF THE INVENTION

Digital communication makes use of various data transmission methodsthat concert information—such as voice or video signal—into bits ofdigital message, and transmit the bits of information after conversion.

The length of the transmitted message is commonly not fixed.

Some solutions consist then in adding to the length information to thetransmitted data. However, in some applications, the data rate is low sothat adding information in the amount of data to be transmitted shouldbe avoided.

An alternative is to use some error correcting codes, which aretransmitted in accompanying the message bits, to determine the lengthinformation at recipient side.

In UMTS, WCDMA, and some other 3G mobile telephony systems, cyclicredundancy check (CRC) bits affixed to the message bits have been widelyapplied to detect the length information of a received message block incertain situations. A mechanism achieving this result is the BlindTransport Format Detection (BTFD).

FIG. 1 illustrates the context where BTFD mechanism may be used. Anemitter 100 comprises a CRC encoder 101. This encoder 101 takes as inputa message of k bits, and adds x bits of CRC code (12 or 16 bits in most3G test cases). These k+x bits are then processed by a convolutionencoder 102 that generates encoded data of k+x+m bits, where m is anumber of zeros padded after CRC-encoded message block to terminate thetrellis of convolutional codes.

The encoded data are then modulated by a modulator 103 and transmittedto a channel 120. This channel being a radio channel, the transmittedsignal is perturbed by noise, so that the receiver 110 receives themodulated frame together with noise.

The receiver includes a demodulator 111 to demodulate the received frameand generate a corresponding demodulated frame. The receiver 110 furtherincludes a decoder 112 so that the demodulated frame can be decoded andthe unknown message length k can be determined accordingly.

The receiver knows only about a set of candidates for the transportformat (i.e. length k of the transmitted messages) from layer-3negotiation, and the problem it faces is to determine the most probabletransport format for the received message.

In other words, turning to the FIG. 2 showing a received message, itconsists in determining where ends the message made of the Data and CRCfields: four candidates are depicted (N=1, 2, 3, 4) and the receivershould determine that the end bit position

Several algorithms have been proposed so far for the demodulator 111 todetermine the right length of the transmitted message so as todemodulate it.

The BTFD solution has been the one standardized by the 3GPP and ETSIorganizations. The TS125.212 document, entitled “Multiplexing andchannel coding (FDD)” describes such an approach and a possiblealgorithm. This solution is illustrated by the FIG. 3.

This method is based on a Viterbi decoder. The correct trellis path ofthe Viterbi decoder ends at the zero state at the correct end bitposition N.

The BTFD approach traces back the surviving trellis path ending at thezero state (hypothetical trellis path) at each possible end bit positionto recover the data sequence. For each recovered data sequence, errordetection is performed by checking the CRC and if there is no error therecovered sequence is declared to be correct.

A variable S(N) is defined as:

$\begin{matrix}{{S(N)} = {{- 10} \times {\log \left( \frac{{a_{0}(N)} - {a_{\min}(N)}}{{a_{\max}(N)} - {a_{\min}(N)}} \right)}}} & (1)\end{matrix}$

Where a_(max)(N) and a_(min)(N) are the maximum and minimum path-metricvalues among all survivors at end bit position N, and a₀(N) is thepath-metric value at zero state.

In a noiseless transmission, S(N) corresponding to the true (i.e. astransmitted) message length k always equal zero. However, in a noisytransmission, it is unlikely that S(N) will equal zero for the truemessage length k. Therefore, the test consists in comparing thisvariable S(N) to a predetermined threshold Δ.

This threshold Δ is called “path selection threshold” and it is a designparameter.

Coming back to the FIG. 3, the algorithm starts with a initializationstep 301 consisting in setting the end bit position N to 1, and two workvariables S_(best)=Δ and n_(end)=0. The variable n_(end) represents thebest end bit position found so far. The variable S_(best) represents thecorresponding best variable S(N) as provided by the Viterbi decoder.

The step 302 consists in determining the variable S(N) for the currentvalue of N, according to the equation (1) given above.

This calculated variable S(N) is then compared with the threshold Δ atstep 303.

If the variable is greater than the threshold, the flow proceeds to step308 to determine whether any additional length candidate exists. Fordoing this, it suffices to check if the current value of N is themaximum number in the set of candidates.

If no, i.e. if there exists at least another candidate, the value of Nis increased by 1 in step 309 and the process loops to step 302.Otherwise, the flow proceeds to step 310 to determine whether the bestsolution n_(end) is equal to zero.

If n_(end)=0, the decoding process ends at step 311 and the receivedmessage is declared to be in error.

If n_(end)>0, then the value of n_(end) is considered to be the bestcandidate for the real length of the received message (step 312).

The other branch of the process starting at step 303 is triggered whenS(N)≦Δ. Then, at a step 304, the Viterbi decoder traces back thereceived message so as to output a block of bits according to thecurrent value of N.

This block of bits is then processed by a CRC checker 305. If the CRC isnot correct, the flow proceeds to the step 308 already explained. If theCRC is correct, then it means that a legitimate message has been found.However, the variable S(N) is to be compared with the currently bestvalue S_(best.), at the step 306, in order to determine if thislegitimate message is better than the previously-found one(s).

If S(N)≦S_(best), the flow proceeds also to step 308.

In the situation where S(N)>S_(best), then a step 307 consists insetting new values to the work variables S_(best) and N_(end), so as:S_(best)=S(N) and N_(end)=N, and the flow also proceed to step 308, forgoing on with following candidates or ending the process.

Nonetheless, as any process based on statistical analysis (here, theViterbi decoding algorithm), the receiver makes errors, with a ratedepending notably on the noise in the transmission channel. Theabove-described approach aims at keeping these decoding errors as low aspossible, but there is still a need to improve the situation byproposing a BTFD process optimizing the decoding performances.

SUMMARY OF THE INVENTION

This is achieved with a method for determining a length of a messageblock of k bits from a length candidate set, received by a receiverthrough a wireless communication channel, comprising steps of:

-   -   Selecting a length candidate among this length candidate set;    -   Decoding a received frame to form a decoded sequence that        includes a message of a length equal to the length candidate, by        a Viterbi decoder;    -   Calculating a Viterbi variable for the length candidate, by the        Viterbi decoder.    -   Comparing this Viterbi variable with a threshold,    -   Repeating the selecting, decoding and calculating steps if the        Viterbi variable is greater than the threshold value, and if        there exists an unselected length candidate among the length        candidate set;    -   If said Viterbi variable is greater than a best value, updating        the best value to the value of the Viterbi variable and updating        a best length to the length candidate;    -   Wherein said threshold is initially set to a value determined        according to the conditions of reception of the receiver.

According embodiments of the invention, this method may also compriseone or several of the following features:

-   -   The best length may be considered as the best estimate of k when        all length candidates of the set have been selected and if the        best length is not null.    -   The threshold may be defined as a function of at least one        criterion representative of the conditions of reception.    -   The threshold may be set to a high value when a criterion is        below a pivot value and to a low value (Δ_(low)) when the        criterion is above this pivot value.    -   An hysteresis approach may be used between a low state where the        threshold is set to a low value and a high state where the        threshold is set to a high value, and the transition from said        high state to said low state is triggered when said criterion is        below a low pivot value (PV_(low)) and the transition from said        low state (S_(L)) to said high state (S_(H)) is triggered when        said criterion is above a high pivot value (PV_(high)).    -   This at least one criterion may comprise a noise-to-signal        ratio.    -   This noise-to-signal ratio can be estimated by a DPCH_Ec/Ioc        parameter.    -   The noise-to-signal ratio can also be estimated by a        signal-to-interference ratio measured on the DPCH channel.    -   The threshold can be fixed.    -   The step of comparing the Viterbi variable with a threshold may        be replaced by a step of comparing said Viterbi variable with        the best value, this best value being initially set to the        threshold (A).    -   The method may also comprise a step of tracing back from the        length candidate, this step being implemented by the Viterbi        decoder.    -   The method may further comprise a step of computing a CRC check        on the block of bits outputted by the Viterbi decoder.

This is also achieved with a computer program product comprising acomputer readable medium, having thereon a computer program comprisingprogram instructions, the computer program being loadable into adata-processing unit and adapted to cause execution of the method whenthe computer program is run by the data-processing unit.

Another object of the invention is a data storage medium having recordedthereon a computer program comprising instructions for performing themethod.

Another object of the invention is a receiver for determining a lengthof a message block of k bits from a length candidate set, receivedthrough a wireless communication channel to which the receiver isconnected, comprising means for:

-   -   Selecting a length candidate among the length candidate set;    -   Decoding a received frame to form a decoded sequence that        includes a message of a length equal to the length candidate, by        a Viterbi decoder of the receiver;    -   Calculating a Viterbi variable for the length candidate, by this        Viterbi decoder.    -   Comparing the Viterbi variable with a threshold,    -   Repeating the selecting decoding and calculating steps if the        Viterbi variable is greater than the threshold value, and if        there exists an unselected length candidate among the length        candidate set;    -   If the Viterbi variable is greater than a best value, updating        this best value to the value of the Viterbi variable and        updating a best length to this length candidate;    -   And wherein the threshold is initially set to a value determined        according to the conditions of reception of the receiver.

Further features and advantages of embodiments of the invention willappear from the following description of some embodiments of theinvention, given as non-limiting examples, with reference to theaccompanying drawings listed hereunder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, already described, shows functional architectures for wirelessemitter and receivers, in which a BTFD can be used.

FIG. 2, already described, illustrates an example for a received dataframe.

FIG. 3, already described, shows a flowchart of a BTFD method accordingto the prior art.

FIG. 4 shows a flowchart according to an embodiment of the invention.

FIG. 5 shows a state diagram related to an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In order to ensure an appropriate behavior of the global system, it isessential to ensure the ability of the blind transport format detection(BTFD) to determine the right transport format (TF) so as the receivedmessages are well decoded by the receiver.

In order to do that, the 3GPP/ETSI document TS 134.121 entitled“Terminal Conformance Specification, Radio Transmission and Reception(FDD) specifies two criteria that shall remain under certain thresholdvalues:

-   -   The block error ratio (BLER)    -   The false format detection ratio (FDR).

The performance, as defined by BLER and FDR values, depends on thegeometry conditions of the receiver, i.e. the DPCH_Ec/Ioc parameter.This parameter is defined by

$\frac{DPCH\_ Ec}{I_{oc}} = {\frac{DPCH\_ Ec}{I_{or}} \times {\frac{I_{or}}{I_{oc}}.}}$

As geometry factor is equivalent to estimate I_(or)/I_(oc) forparticular physical channel DPCH, it corresponds to

$\frac{DPCH\_ Ec}{I_{oc}}.$

These geometry conditions have direct impact on the receptionconditions, i.e. on a signal-to-noise ratio of the receiver.

This

$\frac{DPCH\_ Ec}{I_{oc}}$

parameter represents the ratio of the transmit energy per pseudo-noisechip of the DPCH (Dedicated Physical Channel) to the total transit powerspectral density at the Node B antenna.

According to an aspect of the invention, a parameter of the BTFD isinitially set according to the reception conditions of the receiver.

It can therefore be set according the geometry conditions of thereceiver, and more specifically according to this DPCH_Ec/Ioc parameter.

Thanks to this, the DPCM will better adapt to the transmissionconditions, e.g. noise, in the transmission channel between the emitterand the receiver. The decoding can then benefit from FDR mitigation inbad transmission conditions, while still preventing any BLER performanceregression.

The FIG. 4 shows a flow chart of an embodiment of the invention.

The first step 401 is an initializing step.

It comprises the selecting a length candidate among a length candidatesset.

This length candidate set may have been negotiated earlier between thetwo parties, the emitter and the receiver. As it is a step known in theart, it will not be further explained here. The length candidates may besuccessive bit lengths (like 7, 8, 9 . . . bits) or not.

More concretely, the first step 401 selects an initial index N in thecardinal of the length candidates set. Typically, this initial index isset to 1, meaning that the first length candidate will be first handled.

The initializing step 401 also comprises setting a best length valueN_(end) to 0. This value means that at the start of the BTFD no solutionhas been found.

The initializing step 401 further comprises setting an initial value toa threshold Δ. This threshold is the one to which the Viterbi variableS(N) of the current length candidate N will be compared.

According to an aspect of the invention, the initial value of thisthreshold is set according to the conditions of reception of thereceiver.

More precisely, the threshold Δ can be defined as a function of one orseveral criteria M representative of these conditions of the receiver.In other words, it is possible to write Δ=f(M). This function can be ofdifferent nature, continuous or not, as it will be explained below insome embodiments.

These criteria can comprise a signal-to-noise ratio (SNR). For instance,this ratio SNR can be estimated by the DPCH_Ec/Ioc parameter as earlierexplained.

The general formula Δ=f(M) can then be instantiated as:

$\Delta = {f\left( \frac{DPCH\_ EC}{Ioc} \right)}$

Several embodiments are possible to determine the threshold according tothe DPCH_Ec/Ioc parameter. In other words, several function f arepossible.

A first embodiment can consist in selecting a value for the threshold Δamong two values Δ_(high), Δ_(low) according to a comparison with pivotvalue PV of the DPCH_Ec/Ioc parameter.

A relationship can be provided by the following equations:

$\begin{matrix}{{{{If}\mspace{14mu} \frac{DPCH\_ EC}{I_{oc}}} < {PV}},{{{then}\mspace{14mu} \Delta} = \Delta_{high}}} & \left( {2a} \right) \\{{{{If}\mspace{14mu} \frac{DPCH\_ EC}{I_{oc}}} > {PV}},{{{then}\mspace{14mu} \Delta} = \Delta_{low}}} & \left( {2b} \right)\end{matrix}$

According to a second embodiment of the invention, a hysteresis approachis used with two pivot values PV_(high) and PV_(low) as illustrated bythe FIG. 5. This approach can be represented by two states S_(H), S_(L).

In a high state S_(H), the threshold Δ is set to a high value Δ_(high);while in a low state S_(L), it is set to a low value Δ_(low).

The transition from the high state S_(H) to the low state S_(L) istriggered by the condition

$\frac{DPCH\_ EC}{I_{oc}} < {PV}_{low}$

The transition from the low state S_(L) to the high state S_(H) istriggered by the condition

$\frac{DPCH\_ EC}{I_{oc}} < {PV}_{high}$

The values to be used for the thresholds Δ_(low), Δ_(high) and the pivotvalues PV, PV_(low) and PV_(high) should be carefully set.

The inventor has made numerous tests from which it has been determinedthat a too-low threshold does not filter out enough wrong lengthcandidates. It implies too much CRC errors and an unacceptable increaseof the FDR (False Format Detection Ratio).

On the other side, increasing too much the threshold may improve the FDRparameter in noisy reception conditions, but it implies losing somepotentially good length candidates especially in good receptionconditions. The BLER parameter may increase even beyond the maximumvalue admissible by the 3GPP standards.

It is therefore important to choose the pivot values so as the excursionof the thresholds is kept within a range of optimal values when BLER andFDR parameters are kept in acceptable range both in good receptionconditions and bad reception conditions.

The numeral values can be determined according to the specificity of thecircuitry used to implement the receiver.

Examples of numeric values used in the context of an actual embodimentof the invention made by the inventor are as follow:

For instance, when a single pivot value is used, its value can be PV=−19dB. The thresholds can be Δ_(low)=0; Δ_(high)=0.7 for examples.

In the hysteresis approach, the values can be PV_(high)=−17 dB andPV_(low)=−20 dB, with thresholds like Δ_(low)=0.2; Δ_(high)=0.8.

According to other embodiments, instead of the DPCH_Ec/Ioc parameter, anestimation of a signal to interference ratio (SIR) is used to determinethe thresholds and the pivot values. Indeed, the DPCH_Ec/Ioc parameteris difficult to be known by the receiver. So, an alternative is to usean estimation that can be measured by receiver's circuitry.

This estimation is a SIR measurement measured on the DPCH channel.

This can be determined in various ways. One possible implementation ofthe SIR estimation on the DPCH channel is given by the followingequation:

${SIR}^{DPCH} = {\frac{256}{sf} \times \left\lbrack \frac{C_{data}}{c\sqrt{{SIR}^{CPICH}}} \right\rbrack^{2}}$

Where:

SIR^(CPICH) is a SIR estimation on the CPICH, Common Pilot Channel. ThisCPICH channel is a physical channel carrying pilot data which are knownby the user equipment (UE) and used to compute the SIR estimation inclassical way by filtering ratio between useful signal estimated power(i.e. total power minus noise power) and estimated interference power.

sf is the spreading factor used in the transmission on the DPCH channel.256 being the spreading factor for the CPICH, the ratio 256/sf is ascaling factor.

c is a constant scaling factor depending on the DPCH spreading factorsf. The dependency can be expressed for instance as: c²=√{square rootover (2^(w))}, in which:

$w = \left\{ \begin{matrix}2 & {{{for}\mspace{14mu} {sf}} \in \left\{ {8,16,32,64} \right\}} \\1 & {{{for}\mspace{14mu} {sf}} = 128} \\0 & {{{for}\mspace{14mu} {sf}} = 256}\end{matrix} \right.$

C_(data) is defined such as (C_(data))² should be equal tototal_slot_power, i.e. to

$\sum\limits_{{received}\; \_ \; {slots}}({soft\_ bits})^{2}$

This SIR measurement can be used in the various embodiments given above,so that the threshold Δ is made dependent of this SIR, as representativeof the conditions of reception of the receiver.

After this initializing step 401, the BTFD comprises a step 402consisting in decoding a received frame to form a decoded sequence. Thisdecoded sequence of bits includes a message of a length equal to thelength candidate N.

This step consists in applying a Viterbi decoding algorithm on thisreceiving frame based on this length candidate, so as to determine theViterbi variable S(N).

In a next step 403, this Viterbi variable S(N) is compared with acurrent best value S_(best). This best value S_(best) has beeninitialized to the threshold Δ in the first step 401. It is thusinitially dependent on the reception conditions of the receiver. It canbe updated at each pass of the algorithm when a better value is found,as it will be explained later on.

This comparison determines whether the currently tested length candidateis a potentially good one or not. If the current S(N) is not below thecurrent best solution S_(best), then it means this is not a potentiallygood enough candidate. The flow goes to the step 407 where it is testedwhether there still exists a length candidate available (a not yetalready selected) in the length candidate set. In the case at least oneunselected length candidate exists, the step 408 consists in selecting anew length candidate among this set. This selection step 408 can simplyconsist in increasing the candidate index N by 1, so as to pick up thenext one in the ordered length candidates' set.

Once a new length candidate selected, the decoding and calculating step402 is repeated.

The loop among the steps 402, 403, 407 and 408 terminates when aselected length candidate N provides a Viterbi variable S(N) above thebest value S_(best) (and then the flow goes on to step 404), or whenthere is not more unselected length candidate in the set. Then, the flowgoes from step 407 to step 409.

This step 409 tests the best length candidate that has been found.Again, we can here face to two situations by assessing the best lengthvalue N_(end). If this value is equal to zero, it means that process hasnot been able to find any possible length candidate. In this situation,the process shall return an error by going to the step 410.

If this best length value N_(end) is not equal to zero, it means that apossible best length candidate has been found. This best length valuecan be outputted in the step 411 as the best estimate of the real numberof bits k of the received message.

Going now back to the step 403, if the current Viterbi variable S(N) isgreater than the best value S_(best) the current solution (based on thecurrent length candidate) is traced back by the Viterbi decoder in thestep 404. This allows outputting a block of bits according to thecurrent value of the length candidate N. This step can be performedaccording to prior art technics about Viterbi decoding and BTFDprocesses.

In a following step 405, this block of bits is then processed by a CRCchecker (Cyclic Redundancy Check). If the CRC is not correct, then itmeans that the current length candidate is not a correct one and theflow loops back to the step 407.

According to the invention, the good selection of the threshold Δreduces the probability that a CRC error happens, and thus of the loopback to step 407.

If the CRC is correct, it means that the currently selected lengthcandidate N is the best one. Therefore, the best value S_(best) and thebest length N_(end) should be updated as follows:

S_(best)=S(N)

N_(end)=N

Then the process loops back to step 407 to determine if another lengthcandidate can be selected or not. The potentially new length candidateswill be compared with the updated best value S_(best). If there is nomore length candidate, the current best length N_(end) will be outputtedas the best estimation of the real length of the received message todecode.

It should be noted that in this embodiment only one test is performed onthe Viterbi variable S(N). This embodiment is optimized to reduce theprocessing time.

However the invention can also apply to other embodiments of the BTFDalgorithm, especially when two subsequent tests are performed as in the3GPP standards, i.e. a comparison with the threshold Δ and a comparisonwith the current best value S_(best).

In other words, according to the BTFD process defined in the 3GPPstandard, in the step 403, the current Viterbi value S(N) is compared tothe threshold Δ. This threshold Δ is fixed and an additional step can beinserted after the CRC check 405 for comparing the current Viterbi valueS(N) with the best value S_(best).

In the previously described embodiment, this step of comparing theViterbi variable S(N) with the threshold Δ is replaced by a step ofcomparing the Viterbi variable with the best value S_(best), which hasbeen initially set to this threshold Δ.

1. A method for determining a length of a message block of k bits from alength candidate set, received by a receiver through a wirelesscommunication channel, comprising steps of: Selecting a length candidateamong said length candidate set; Decoding a received frame to form adecoded sequence that includes a message of a length equal to saidlength candidate, by a Viterbi decoder; Calculating a Viterbi variablefor said length candidate, by said Viterbi decoder. Comparing saidViterbi variable with a threshold, Repeating the selecting, decoding andcalculating steps if said Viterbi variable is greater than saidthreshold value, and if there exists an unselected length candidateamong said length candidate set; If said Viterbi variable is greaterthan a best value, updating said best value to said Viterbi variable andupdating a best length to said length candidate; Wherein said thresholdis initially set to a value determined according to the conditions ofreception of said receiver.
 2. A method according to claim 1, whereinsaid best length is considered as the best estimate of k when all lengthcandidates of said set have been selected and if said best length is notnull.
 3. A method according to claim 1, wherein said threshold isdefined as a function of at least one criterion representative of saidconditions of reception.
 4. A method according to claim 3, wherein saidthreshold is set to a high value when said at least one criterion isbelow a pivot value and to a low value when said at least one criterionis above said pivot value.
 5. A method according to claim 6, wherein ahysteresis approach is used between a low state where said threshold isset to a low value and a high state where said threshold is set to ahigh value, and wherein the transition from said high state to said lowstate is triggered when said at least one criterion is below a low pivotvalue and the transition from said low state to said high state istriggered when said at least one criterion is above a high pivot value.6. A method according to claim 3, wherein said at least one criterioncomprises a noise-to-signal ratio.
 7. A method according to claim 6,wherein said noise-to-signal ratio is estimated by a DPCH_Ec/Iocparameter.
 8. A method according to claim 6, wherein said anoise-to-signal ratio is estimated by a signal-to-interference ratiomeasured on the DPCH channel.
 9. The method of claim 1, wherein saidthreshold is fixed.
 10. The method of claim 1, wherein the step ofcomparing said Viterbi variable with a threshold is replaced by a stepof comparing said Viterbi variable with said best value, said best valuebeing initially set to said threshold.
 11. The method of claim 1,further comprising a step of tracing back from said length candidate bysaid Viterbi decoder.
 12. The method of any of claim 1, furthercomprising a step of computing a CRC check on the block of bitsoutputted by said Viterbi decoder.
 13. A computer program productcomprising a computer readable medium, having thereon a computer programcomprising program instructions, the computer program being loadableinto a data-processing unit and adapted to cause execution of the methodaccording to claim 1 when the computer program is run by thedata-processing unit.
 14. Data storage medium having recorded thereon acomputer program comprising instructions for performing the methodaccording to claim
 1. 15. A receiver for determining a length of amessage block of k bits from a length candidate set, received through awireless communication channel to which said receiver is connected,comprising means for: Selecting a length candidate among said lengthcandidate set; Decoding a received frame to form a decoded sequence thatincludes a message of a length equal to said length candidate, by aViterbi decoder of said receiver; Calculating a Viterbi variable forsaid length candidate, by said Viterbi decoder; Comparing said Viterbivariable with a threshold, Repeating the selecting, decoding andcalculating steps if said Viterbi variable is greater than saidthreshold value, and if there exists an unselected length candidateamong said length candidate set; If said Viterbi variable is greaterthan a best value, updating said best value to said Viterbi variable andupdating a best length to said length candidate; and Wherein saidthreshold is initially set to a value determined according to theconditions of reception of said receiver.